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Project description

Quantifying the risk of pitch canker to susceptible pines in California. (03XU022)
Program Exotic Pests and Diseases Research Program
Principal
investigator
T.R. Gordon, Plant Pathology, UC Davis
Host/habitat Monterey Pine; Pines
Pest Pitch Canker Fusarium circinatum
Discipline Plant Pathology
Review
panel
Urban Systems
Start year (duration)  2003 (Three Years)
Objectives Characterize the conditions of temperature and moisture required for infection by Fusarium circinatum.

Develop a mathematical model to predict infection frequency based on temperature and relative humidity.

Validate the model under field conditions.

Project
Summary
Pitch canker is a serious invasive disease affecting pines in both urban and native forests. Susceptible species are found throughout the state. However, the distribution of the disease and previous experimental work suggests that the risk to susceptible pines may be influenced by significant climatic limitations on the activity of the pathogen. This project would provide the information needed to confirm and quantify these limitations and thereby assist regulators and land managers in more accurately assessing the risk of pitch canker.
Final report The results of our experiments show that: 1) shallow wounds, similar in size to those made by insects, are infected at a lower frequency than deeper wounds, 2) the likelihood of infection of both deep and shallow wounds is affected by available moisture, and 3) infection frequency is influenced by ambient temperature and relative humidity during the first 72 hours following inoculation. Specifically, the likelihood of infection increases with the number of hours above 10°C, provided relative humidity is above 50%. This result is consistent with the facts that 1) spore germination occurs very slowly at 10°C and progressively more rapidly as temperature increases up to 25°C, and 2) wounds become less susceptible over time as documented by a significant drop in infection frequency two days after a wound is made. Thus, the success of an infection appears to be critically dependent on the length of time required for spores to germinate, which is dictated to a significant extent by temperature and moisture. This supports the hypothesis that pitch canker is a climate-limited disease and that it is unlikely to become a major problem in northern and inland forests, where conditions are either too dry and/or too cool to support a high rate of infection.

Third-year
progress
We have conducted a series of experiments designed to determine what conditions of temperature and relative humidity are required for Fusarium circinatum, the cause of pitch canker, to establish infections. The results of experiments to date have revealed that: 1) shallow wounds, similar in size to those made by insects, are more prone to environmental limitations than larger wounds and 2) low temperatures in winter significantly reduce the likelihood of infections.

Second-year
progress
The purpose of this project was to better characterize the impacts of pitch canker so that landowners could make more informed decisions about tree removal. To this end, we have characterized the susceptibility of trees in 16 locations (plots): eight in areas where pitch canker was well established (old areas) and eight in areas where the disease was a more recent occurrence (new areas).

In 2002, 20 trees in each plot were tested for susceptibility by making an artificial inoculation on three branches of each tree. The results showed that trees in areas where pitch canker has been present for a long time tend to be less susceptible to the disease than trees in areas where pitch canker has only recently become established. Similar results were obtained when the same trees were inoculated in 2004, but the magnitude of the difference between areas was less than in 2002.

In a separate study, we gauged the susceptibilities of trees that were once severely diseased but were subsequently free of pitch canker (=disease remission). A total of 82 trees in seven plots were tested, and nearly all were highly resistant to pitch canker. These results are indicative of induced resistance, something that has not previously been demonstrated in either hardwood or coniferous trees. Induced resistance appears to be a critical component of disease remission and can be expected to moderate the impact of pitch canker on susceptible pines throughout California. This information has been used to update the guidelines on tree replacement that were originally prepared and disseminated by the California Pitch Canker Task Force.

First-year
progress
We have initiated a series of experiments designed to determine what conditions of temperature and relative humidity are required for Fusarium circinatum, the cause of pitch canker, to establish infections. The results of these experiments will reveal the extent to which climate limits the ability of this fungus to cause disease. This information will be used to assess the risk of pitch canker to susceptible species throughout the state of California.

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